Amino acid-derived compounds as effective corrosion inhibitors for mild steel in hydrochloric acid: Gravimetric, electrochemical and computational studies

Mild Steel (MS) is widely used in industry for its mechanical strength and low cost. However, the material is very sensitive to corrosion, especially under acid descaling using 15 % hydrochloric acid (HCl). Two new amino acid-based corrosion inhibitors, INH-1 and INH-2, are developed in this work that offer very effective and environmentally friendly corrosion protection. At the optimum concentration of 200 μM, INH-1 has an inhibition efficiency of 99.3 %, whereas INH-2 has 94.6 %, far surpassing many traditional inhibitors. The corrosion rates of the MS samples were obtained through gravimetric analysis and it is found that the corrosion rate of MS in uninhibited 15 % HCl at 303 K is 40.2 mm/y, which is significantly decreased to 0.265 mm/y by INH-1 and 2.15 mm/y by INH-2. Electrochemical impedance spectroscopy (EIS) analysis reveals a significant rise in charge transfer resistance (R_ct) from 5 Ω cm² (blank) to 149 Ω cm² for INH-1 and 51 Ω cm² for INH-2, reflecting strong protection of the surface. The inhibitors exhibit Langmuir adsorption isotherm behavior with ΔG_ads values of −37.38 kJ mol⁻¹ for INH-1 and −37.05 kJ mol⁻¹ for INH-2. This observation establishes a prevailing physisorption mechanism of the corrosion inhibitors. X-ray photoelectron spectroscopy (XPS) indicates the development of a stable organic-inorganic hybrid protective layer with the presence of carbon, nitrogen, sulfur, iron, and oxygen, which inhibits metal dissolution effectively. These results are supplemented by theoretical work based on density functional theory (DFT) and Monte Carlo simulations, exhibiting a low energy gap (ΔE) and high electron-donating ability that facilitate adsorption on the steel surface. The novel molecular structure of these inhibitors, featuring the combination of multiple heteroatoms and aromatic systems, provides better resistance to corrosion even at higher temperatures up to 333 K. The exceptional effectiveness at elevated temperatures of INH-1 and INH-2 reveal their prospects as non-polluting, high-performance substitutes for corrosion protection.